In a semiconductor device manufacturing, a semiconductor wafer or a glass substrate for an LCD (hereinafter referred to as “substrate”) is subjected to a series of process steps including: coating the substrate with a resist solution to form a resist film; transferring a circuit pattern, with scale reduction, to the resist film by using photolithography; developing the resist film; and thereafter removing the resist film from the substrate.
In view of the growing emphasis on environmental conservation in recent years, there has been recently proposed a resist-film removing method, which is free of problems relating to waste-liquid treatment and which uses ozone (O3) and water vapor. In such a method, an ozone generator of a silent discharge type is generally used as ozone generating means, as described in the “Background Art” section of JP1-282104A. An ozone generator of a silent discharge type includes an ozone generating cell having a pair of electrodes and a dielectric body arranged between the electrodes, and generates ozone by passing oxygen through a discharging space formed by applying a high alternating voltage across the electrodes.
A dielectric body composed of a ceramic coating with Cr additive is often used in an ozone generator of a discharge type. In this case, Cr in the ceramic is sputtered by discharge energy, and is contained in ozone gas. If a substrate is processed with ozone gas containing Cr, metallic contamination of a substrate by Cr may possibly occur.
An ozone generator of a discharge type generates ozone gas with the use of oxygen (O2) gas containing nitrogen (N2) gas in order to improve ozone-gas generating efficiency. Thus, the generated ozone gas contains nitrogen oxides (NOx). NOx reacts with moisture to form nitric acid. If a processing vessel is formed of a stainless steel, nitric acid dissolves the processing vessel so that metallic components such as Cr elute from the processing vessel. The metallic components may possibly cause metallic contamination of a substrate.
The above metallic contamination is insignificant. However, the best possible reduction in such metallic contamination is desired in view of higher integration of semiconductor devices in the future.
JP8-59214A discloses an ozone generator equipped with a NOx removing means, which is composed of a container containing zeolite as a NOx adsorbent; or composed of a container containing pure water into which NOx contained in ozone gas is dissolved through a bubbling operation.
However, the NOx removing means disclosed in JP8-59214A can not achieve a sufficient NOx removing efficiency.